The introduction of genes into eukaryotic cells as autonomously replicating and transcribing plasmid expression vectors presents a more facile approach to the regulation of gene expression. The gene copy number is amplified, the gene flanking sequences remain constant and the genes can be selectively mutated by genetic engineering. The basic problems with this approach are as follows: (1) only a very small percentage of cells (1-10%) are transformed by exogenous DNA; (2) increasing gene copy number does not result in increased cellular levels of that protein (eg, human Alpha- and Beta-globin, herpes thymidine kinase); and (3) unless the gene product is easily detected by fluorescent antibody, there is no way to quantitate the percentage of cells transformed. This project attempts to address these problems using SV-40 expression vectors introduced into COS-1 cells. Studies with liposome encapsidated DNA suggest that mitotic cells are more efficiently transformed, therefore the cell cycle dependence of COS-1 cell transformation is being studied. Increased transformation efficiency of COS-1 cells will facilitate the study of the regulatory mechanisms involved in gene copy number vs. gene product expression. A DNA hybridization assays if being developed to quantitate the fraction of transformed COS-1 cells.